Determination of a ground receiver position

ABSTRACT

Technology for determining a geographical location of a ground receiver is disclosed. A plurality of radio frequency (RF) signals from a plurality of RF signal carriers may be received at the ground receiver. The plurality of RF signal carriers may include satellites operated by a foreign entity or non-global positioning system (non-GPS) satellites. The ground receiver may measure a Doppler shift associated with each of the plurality of RF signals. The geographical location of the ground receiver may be determined in X, Y and Z coordinates based in part on the Doppler shift associated with each of the plurality of RF signals.

RELATED APPLICATIONS

This is a divisional application of U.S. patent application Ser. No.15/610,597, filed on May 31, 2017, which is a divisional application ofU.S. patent application Ser. No. 14/498,069, filed on Sep. 26, 2014,each of which is incorporated by reference herein in its entirety.

BACKGROUND

Navigation systems, such as GPS, enable a receiver to determine alocation from ranging signals received from a plurality of satellites.The ranging signals can be broadcasted on frequencies, such as the L1signal (1.57542 gigahertz [GHz]), L2 signal (1.2276 GHz), and/or L5signal (1.17645 GHz). L1 can have a wavelength of about 19 centimeters(cm) and L2 can have a wavelength of about 24 cm. Position can bedetermined from code and/or carrier phase information. A code divisionmultiple access (CDMA) code is transmitted by the GPS satellites to thereceiver and correlated with replica codes to determine ranges todifferent satellites, which can be used to determine the position of aGPS receiver on or near the Earth. Generally, a GPS receiver receivessignals from multiple GPS satellites (e.g., four GPS satellites) to findits position.

It is desirable for receivers to be able to locate itself when no GPSsignals are available or only a single such signal may be received. Awide array of techniques have been proposed to provide such positioninformation including the use of stellar observations, inertialmeasurement units, signals of opportunity such as TV and cell phonesignals. Each of these techniques suffers an issue of positionalaccuracy or lack of availability at times. For military operations, itis desirable to operate in all weather conditions, including clouds(which may obscure stellar measurements), and without reliance on TV orcellular signals in the adversaries country.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of the disclosure will be apparent from thedetailed description which follows, taken in conjunction with theaccompanying drawings, which together illustrate, by way of example,features of the disclosure; and, wherein:

FIG. 1 illustrates a diagram of a plurality of navigation satellites, aplurality of low Earth orbit (LEO) satellites, and a plurality ofgeostationary Earth orbit (GEO) satellites or high Earth orbit (HEO)satellites in accordance with an example;

FIG. 2 illustrates a diagram of a ground receiver receiving a pluralityof radio frequency (RF) signals from a plurality of RF signal carriersand determining a geographical location of the ground receiver based ona Doppler shift associated with each of the plurality of RF signals inaccordance with an example;

FIG. 3 illustrates a diagram of a ground receiver receiving a pluralityof radio frequency (RF) signals and global positioning system (GPS)signals from a plurality of RF signal carriers and GPS satellites,respectively, and determining a geographical location of the groundreceiver based on a Doppler shift associated with each of the pluralityof RF signals and GPS signals in accordance with an example;

FIG. 4 illustrates a diagram of a ground receiver using radio frequency(RF) signals received directly from an RF signal carrier, transponded RFsignals received from the RF signal carrier via a relay satellite, andtransponded global positioning system (GPS) signals received from a GPSsatellite via the relay satellite in order to determine a geographicallocation of the ground receiver in accordance with an example;

FIG. 5 illustrates a diagram of a ground receiver using radio frequency(RF) signals received directly from an RF signal carrier and transpondedRF signals received from the RF signal carrier via a relay satellite inorder to determine a geographical location of the ground receiver inaccordance with an example;

FIG. 6 depicts a flow chart of a method for determining a geographicallocation of a ground receiver in accordance with an example;

FIG. 7 depicts a flow chart of an additional method for determining ageographical location of a ground receiver in accordance with anexample; and

FIG. 8 depicts functionality of computer circuitry of a ground receiveroperable to determine a geographical location of the ground receiver inaccordance with an example;

Reference will now be made to the exemplary embodiments illustrated, andspecific language will be used herein to describe the same. It willnevertheless be understood that no limitation of the scope of theinvention is thereby intended.

DETAILED DESCRIPTION

Before the present invention is disclosed and described, it is to beunderstood that this invention is not limited to the particularstructures, process steps, or materials disclosed herein, but isextended to equivalents thereof as would be recognized by thoseordinarily skilled in the relevant arts. It should also be understoodthat terminology employed herein is used for the purpose of describingparticular examples only and is not intended to be limiting. The samereference numerals in different drawings represent the same element.Numbers provided in flow charts and processes are provided for clarityin illustrating steps and operations and do not necessarily indicate aparticular order or sequence.

EXAMPLE EMBODIMENTS

An initial overview of technology embodiments is provided below and thenspecific technology embodiments are described in further detail later.This initial summary is intended to aid readers in understanding thetechnology more quickly but is not intended to identify key features oressential features of the technology nor is it intended to limit thescope of the claimed subject matter.

FIG. 1 illustrates a constellation of satellites 120A-J that orbit theEarth 110. The satellites 120A-J can include low Earth orbit (LEO)satellites, medium Earth orbit (MEO) satellites, high Earth orbit (HEO)satellites or geostationary Earth orbit (GEO) satellites. The satellites120A-J can include cube satellites (CUBESATs), pico-satellites,nano-satellites, or micro-satellites, wherein the CUBESATs weigh lessthan 5 kilograms, or others as recognized by those skilled in the art.In one example, the satellites 120A-J can be dedicated navigationsatellites that can enable users to locate their position by decodingsignals received from the satellites 120A-J.

In one example, the satellites 120A-J can be radio frequency (RF) signalcarriers. In one example, RF signal carriers can also be referred to ascarriers of opportunity (COOPs) because these satellites provide RFsignals whose measured carrier frequency and known location can be usedfor position determination of a ground receiver, as described in greaterdetail below.

In one aspect, the term “RF signal carrier” can refer to both GPSsatellites and non-dedicated satellites.

The term “non-dedicated satellites” can refer to satellites, such asthose not dedicated to GPS related navigational purpose applications(non-GPS satellites). The non-dedicated satellites can be owned oroperated either domestically or by a foreign entity. Examples ofnon-dedicated satellites can include weather satellites, communicationsatellites, research satellites, or reconnaissance satellites operatedby foreign nations. In another aspect, non-dedicated satellites caninclude navigation systems operated by foreign countries. Examples ofsuch systems include, but are not limited to, Global NavigationSatellite System (GLONASS) operated by Russia, Galileo operated by theEuropean Space Agency (ESA), Indian Regional Navigational SatelliteSystem (IRNSS) operated by India, and Beidou-2 operated by China.Alternatively, the term “RF signal carrier” can refer to dedicatedsatellites operated domestically, such as GPS satellites.

LEO can generally be defined as an orbit within the locus extending fromthe Earth's surface 110 up to an altitude of approximately 2,000kilometers (km). MEO can be a region of space around the Earth above theLEO (altitude of approximately 2,000 km or 1,243 miles (mi)) and belowgeostationary orbit (altitude of 35,786 km or 22,236 mi). Thegeostationary orbit, also known as the geostationary Earth orbit (GEO),can have a period approximately equal to the Earth's rotational periodand an orbital eccentricity of approximately zero. The GPS satellitescan be low Earth orbit (LEO), medium Earth orbit (MEO) satellites, orgeostationary Earth orbit (GEO) satellites.

A GEO satellite can appear motionless (or slow-moving), at a fixedposition in the sky, relative to ground observers. The GEO satellite canappear motionless to ground observers because the orbit of the GEOsatellite is substantially similar to that of the Earth's rotationalperiod. A GEO satellite can have a near 24 hour orbit, or an orbit nearthe rotational rate of the Earth, hence geostationary Earth orbit (GEO).In an example, a MEO satellite can have an orbital altitude ofapproximately 20,000 km with a near 12 hour orbit. The signal from a GPSsatellite can be decoded to generate an estimate of range from thereceiver to the transmitting GPS satellite.

The GPS signal can be a spread-spectrum, pseudo random noise (PRN)signal that encodes the location of the transmitting satellite. In anexample, the GPS signals can use the L1, L2, or L5 frequency band. EachGPS signal can be used to solve for one unknown in the three dimensional(3D) position of the ground receiver (e.g., GPS receiver). The terms“ground receiver” and “ground station” may be used interchangeablyherein. Since a 3D position can have three unknowns, three independentsignals from three separate GPS satellites can be used to calculate a 3Dposition. Since the GPS satellite and the GPS receiver do not operateusing a same clock, a fourth independent signal from a fourth GPSsatellite can be used to compensate for clock bias in the GPS receiver.So, measurements from the independent GPS signals from four differentGPS satellites can be used to calculate a precise GPS receiver position.Sometimes more than four satellites may be in view of a GPS receiver, soadditional GPS signals can provide redundancy or additional errorchecking for the measurements used to calculate the GPS receiverposition.

In one example, the position (e.g., in X, Y and Z dimensions) of theground receiver can be determined using the measured Doppler shift fromone or more non-dedicated satellites. The X dimension can refer tolatitude, the Y dimension can refer to longitude, and the Z dimensioncan refer to an altitude. The Doppler shift can be a change in afrequency of a wave for an observer, such as a receiver, moving relativeto the source of the wave, such as a transmitter on a satellite. Themotion of the observer, the source, or both can generate a change of thefrequency. When the source of the waves is moving toward the observer,each successive wave crest is emitted from a position closer to theobserver than the previous wave. Therefore, each wave takes slightlyless time to reach the observer than the previous wave. Thus, the timebetween the arrivals of successive wave crests at the observer isreduced, causing an increase in the perceived frequency. Conversely, ifthe source of waves is moving away from the observer, each wave isemitted from a position farther from the observer than the previouswave, so the arrival time between successive waves is increased,reducing the perceived frequency.

Objects moving at greater velocities relative to each other can providelarger size Doppler measurements than objects moving at slowervelocities relative to each other.

A typical GPS receiver can determine its position when having aline-of-sight to at least four GPS satellites. In other words, the GPSreceiver can determine its position when receiving at least four GPSsignals from the four GPS satellites. However, there are areas where GPSis unavailable or degraded (e.g., mountain ranges, remote areas). Inthese situations, the GPS receiver is generally unable to determine itsposition.

As described herein, a ground receiver or ground station can use GPSsignals and/or other types of RF signals in order to determine itsposition. The ground receiver can be stationary, moving, sailing orflying. As a result, users can be provided with an alternative for GPScapability. The ground receiver can receive RF signals transmitted fromsatellites. The receiver can know the original frequency of transmissionof each RF signal carriers or COOPs (i.e., satellites orbiting theEarth) so it may compute the Doppler Shift and trend in Doppler shift.Using Doppler shift can avoid the decoding of signals or extractinginformation from radio signal internals. The Doppler shift can bemeasured with a single antenna. The ground receiver can use the Dopplershift and trend in Doppler shift to determine its position. In oneexample, the ground receiver can determine its position using at leastfour RF signals. In addition, the ground receiver can use a carrierfrequency associated with the RF signal and knowledge of the ephemerisof the satellite transmitting the RF carrier when determining itsposition.

FIG. 2 illustrates a diagram of a ground receiver 240 receiving aplurality of radio frequency (RF) signals from a plurality of RF signalcarriers (COOPs) 212A-D and determining a geographical location of theground receiver 240 using the plurality of RF signals. In general, alarge number of satellites that are orbiting the Earth 250 transmit RFsignals. Although many of these RF signals are not generally used fornavigation purposes, the ground receiver 240 can utilize these RFsignals for navigation purposes, in addition to or as an alternative toGPS signals that are intended for navigation purposes.

The RF signal carriers 212A-D can include satellites that are operatedor owned by a foreign entity (e.g., navigation satellites operated orowned by a foreign country) or non-GPS satellites (e.g., weathersatellites, communication satellites, research satellites, orreconnaissance satellites). Therefore, in one example configuration, theRF signals are not GPS signals or ranging signals. Rather, the RFsignals can include RF signals intended for weather, communication,research, etc. In addition, the RF signals can be available fromUltra-high frequency (UHF) to Ka band at signal levels commensurate withlow gain antennas. UHF designates a radio frequency range between 300megahertz (MHz) and 3000 MHz (or 3 gigahertz (Ghz)). The Ka band coversthe frequencies of 26.5-40 GHz.

The RF signal carriers 212A-D may be in LEO orbit, MEO orbit and/or GEOorbit. The RF signal carriers 212A-D in LEO orbit can includecommunication satellites, such as IRIDIUM, GLOBALSTAR and ORBCOMM. TheRF signal carriers 212A-D in MEO orbit can include navigationsatellites, such as Galileo and GLONASS. The RF signal carriers 212A-Din GEO orbit can include communication satellites, such as ultra-highfrequency (UHF) Follow-On and mobile user objective system (MUOS). TheRF signal carriers 212A-D can be referred to as “carriers ofopportunity” because these satellites provide RF signals that enable theground receiver 240 to determine its location.

The ground receiver 240 is a broadband receiver that can receive aplurality of RF signals from the RF signal carriers 212A-D. The groundreceiver 240 can know source locations associated with the RF signalcarriers 212A-D. The ground receiver 240 can identify a carrierfrequency of a transmitter (i.e., the RF signal carriers 212A-D). Inother words, the ground receiver 240 can identify a carrier frequencyfor each of the RF signals communicated from the RF signal carriers. Theground receiver 240 can identify a modulation format associated with thecarrier frequency. In addition, the ground receiver 240 can identify ormeasure an orbit (e.g., ephemeris) of the RF signal carriers. The groundreceiver 240 can previously identify the carrier frequency, modulationformat, and ephemeris information based on an almanac or other externalsources, or by measuring this. The ground receiver 240 can know thefrequencies transmitted by the RF signal carriers 212A-D and the orbitsof the RF signal carriers 212A-D. If the ground receiver 240 isdirectional, the ground receiver 240 can be pointed such that the RFsignal is received. The RF signal (e.g., a GPS signal) can be measured,demodulated, decoded, and the information from the decoding is processedto obtain a pseudo-range. The information from the decoding can includea measured Doppler shift and an orbital location of each transmitter.

In one example, the ground receiver 240 generally does not knowinformation conveyed in the RF signals other than the source location,carrier frequency and/or modulation format associated with the RFsignals. Therefore, if the RF signals are conveying other types ofinformation, such as weather information, the ground receiver 240 canignore these other types of information because determination of theground receiver's position generally does not require knowledge or useof such information.

In one example, the ground receiver 240 can know whether the RF signalshave a spread-spectrum type modulation, and if so, the ground receiver240 can identify a chip rate of the RF signals. The chip rate of a codeis a number of pulses per second (i.e., chips per second) at which thecode is transmitted or received.

The ground receiver 240 can measure a Doppler shift of each of the RFsignals. In particular, the ground receiver 240 can measure a frequencyof each RF signal that is not spread-spectrum received at the groundreceiver 240 from the RF signal carriers 212A-D. The ground receiver 240can compare the measured frequency to frequency values in the almanac.In other words, the almanac can include predetermined frequency valuesfor each of the RF signals according to the ephemeris of the RF signalcarriers 212A-D associated with the RF signals. The ground receiver 240can calculate a difference in the measured frequency and the frequencyvalues in the almanac in order to determine the Doppler shift of the RFsignal.

If the RF signals received at the ground station 240 are spread-spectrumsignals, the ground receiver 240 generally cannot measure the Dopplershift until the RF signals are despread. For example, the groundreceiver 240 can perform codeless despreading or coded despreading withrespect to the spread-spectrum RF signals before determining the Dopplershift.

The ground receiver 240 can measure the Doppler shift for at least fourRF signals received from the RF signal carriers 212A-D. The groundreceiver 240 can estimate its position (e.g., in X, Y and Z coordinates)using the Doppler shift for the at least four RF signals. In addition,the ground receiver 240 can determine an oscillator offset associatedwith the ground receiver 240. A local oscillator in the ground receiver240 for performing RF measurements can drift. The ground receiver 240can calculate the oscillator offset in order to estimate the drift ofthe local oscillator. Therefore, the ground receiver 240 can use theDoppler shift to estimate X, Y, Z and receiver oscillator offset. In oneexample, RF signals received from RF signal carriers 212A-D in LEO orbitcan enable the ground receiver's position to be determined more rapidlyas compared with using RF signals with less Doppler shifts ortransmitted at greater distances (e.g., RF signals communicated fromsignal carriers 212A-D in GEO and HEO orbits).

In an alternative example, the ground receiver 240 can use techniquesother than measuring the Doppler shift when determining its position.For example, the ground receiver 240 can use angle of arrival, timedifference of arrival (TDOA) or frequency difference of arrival (FDOA)of the RF signals received from the RF signal carriers 212A-D and thenuse this information to determine the ground receiver's position.However, using angle of arrival, TDOA and FDOA can require two or moreantennas, whereas using Doppler shift can require a single antenna.

FIG. 3 illustrates a diagram of a ground receiver 340 receiving aplurality of radio frequency (RF) signals and global positioning system(GPS) signals from satellites 310A-F. The satellites 310A-F can includea plurality of RF signal carriers and GPS satellites. The RF signalcarriers can include satellites that are operated by a foreign country(e.g., navigation satellites) or non-GPS satellites (e.g., weathersatellites, communication satellites, research satellites, orreconnaissance satellites). Therefore, in one configuration, the signalscommunicated from the satellites 310A-F can include GPS signals and/ornon-GPS signals (e.g., RF signals). The non-GPS signals can includenon-ranging RF signals. In addition, the satellites 310A-F may be in LEOorbit, MEO orbit and/or GEO orbit.

In one example, the ground receiver 340 can receive a plurality of GPSsignals from one or more GPS satellites. The ground receiver 340 candetermine a geographical location of the ground receiver 310 using aDoppler shift associated with the plurality of GPS signals. In otherwords, the ground receiver 340 can determine its position in X, Y and Zcoordinates (i.e., latitude, longitude and altitude) using the Dopplershift of the GPS signals. In addition, the ground receiver 340 canreceive a plurality of RF signals from one or more RF signal carriers.The ground receiver 340 can measure the Doppler shift associated witheach of the plurality of non-GPS RF signals. The ground receiver 340 candetermine an orbit (i.e., ephemeris) for the plurality of RF signalcarriers using the Doppler shift and Doppler trend associated with theRF signals and the geographical location of the ground receiver (asdetermined using the Doppler shift of the GPS signals). For example,prior to entering an area of GPS jamming, the ground receiver 340 canupdate the ephemeris for each RF signal carrier using the RF signals andthe ground receiver's GPS-derived location as truth. When the GPSsignals become unavailable or jamming is detected, the ground receiver340 can continue to navigate itself using the RF signals from the RFsignal carriers.

In one configuration, the ground receiver 340 can receive at least oneGPS signal from a GPS satellite. In addition, the ground receiver 340can receive at least one RF signal from an RF signal carrier. The groundreceiver 340 can determine the geographical position of the groundreceiver 340 using the at least one GPS signal and the at least one RFsignal. In other words, the ground receiver 340 can use a combination ofRF signals and GPS signals when, for example, less than four GPS signalsand/or less than four RF signals are in line-of-sight with the groundreceiver 340. As an example, the ground receiver 340 can use two GPSsignals and two RF signals to determine its position.

In one example, the ground receiver 340 can receive at least four GPSsignals from GPS satellites and determine its geographical locationusing a Doppler shift associated with the four GPS signals. In addition,the ground receiver 340 can receive at least four RF signals from RFsignal carriers and determine its geographical location using a Dopplershift associated with the four RF signals. The ground receiver 340 cancompare the geographical location determined using the GPS signals withthe geographical location determined using the RF signals in order toverify that the ground receiver 340 is not receiving spoofed GPS signalsfrom the GPS satellites. Therefore, the ground receiver 340 can monitora signal strength of the GPS signals and switch to an RF-signal-onlyreceiving mode if the signal strength changes or upon detecting otherindications that the GPS signals could be jammed or spoofed.

FIG. 4 illustrates a diagram of a ground receiver 440 using radiofrequency (RF) signals received directly from an RF signal carrier 430,transponded RF signals received from the RF signal carrier 430 via arelay satellite 420, and transponded global positioning system (GPS)signals received from a GPS satellite 410 via the relay satellite 420 inorder to determine a geographical location of the ground receiver 440.

The geographical location of the ground receiver 440 can be determinedusing a range from a transmitting satellite (e.g., the RF signal carrier430) and the ground receiver 440. The determination of the range can beuseful when the RF signal carrier 430 is a GEO satellite, as there islittle or no Doppler shift available. In addition, the relay satellite420 may be responsible for conversion of a Doppler signal to a rangingsignal, which can be used to determine the geographical location. Thespeed of convergence using range information is faster for GEO and MEOsatellites as compared to using Doppler shift only. In one example,ranges to a plurality of transmitting satellites can be determined.

The RF signal carrier 430 can include a satellite operated by a foreignentity or a non-GPS satellite. The ground receiver 440 can correlateeach direct RF signal with each transponded RF signal in order to obtaina time lag of the transponded RF or transponded GPS signal with respectto the direct RF signal. The ground receiver 440 can determine a Dopplershift for each of the signals (e.g., direct RF signals, transponded RFsignals, transponded GPS signals) received at the ground receiver 440.The ground receiver 440 can determine ranges to each of the GPSsatellite 410, the relay satellite 420 and the RF signal carrier 430using the RF signals, GPS signals and transponded RF signals. The groundreceiver 440 can use that range information, time lag information and/orDoppler shift information to determine the ground receiver's location.

In one example, the RF signal carrier 430 and the GPS satellite 410 canbe GEO satellites. In general, GEO satellites are the most commonlyfound space-based transmitters. However, RF signals from GEO satelliteshave the smallest Doppler shift of any satellites (e.g., LEO or MEOsatellites) when received at a terrestrial receiver (i.e., the groundreceiver 440). The Doppler shift can be approximately zero when theground receiver 440 is at rest. A higher Doppler shift can enable theground receiver 440 to determine its position more rapidly as comparedwith a lower Doppler shift. In one example, the RF signals are not GPSsignals or ranging signals.

In order to increase the Doppler shift of RF signals communicated by theRF signal carrier 430, the relay satellite 420 can transpond the RFsignal communicated from the RF signal carrier 430, thereby increasingthe Doppler shift. By transponding the RF signal (i.e., shifting afrequency associated with the RF signal), the relay satellite 420 cansimultaneously impart a larger Doppler shift to the RF signal. As aresult, the larger Doppler shift can result in the ground receiver 440being able to more rapidly determine its location. In one example, theground receiver 440 does not know specific types of information (e.g.,weather information) conveyed in the RF signal from the RF signalcarrier 430 other than a carrier frequency of the RF signal and anephemeris of the relay satellite 420 augmenting the Doppler shift in theRF signal.

In one configuration, the relay satellite 420 can be a cube satellite(CUBESAT), pico-satellite, nano-satellite, micro-satellite, or othersimilar types of small and/or inexpensive satellites. The CUBESAT can bea type of miniaturized satellite that can have a volume of approximatelya liter (10 centimeter (cm) cube) with a weight less than 2 kilograms(kg). The CUBESAT can use commercial off-the-shelf electronicscomponents. The picosatellite (or picosat) can refer to an artificialsatellite with a wet mass between 0.1 and 1 kg (0.22 and 2.2 lb). Thenanosatellite (or nanosat) can refer to an artificial satellite with awet mass between 1 and 10 kilograms (kg) (2.2 and 22 pounds (lb)). Amicrosatellite (or microsat) can refer to an artificial satellite with awet mass between 10 and 100 kg (22 and 220 lb).

The relay satellite 420 can include various components capable ofproviding various functions, such a power source or a power generationmechanism, a mechanism to control heating and cooling of the relaysatellite 420, and/or a mechanism to point a transmitter or antenna tothe Earth. The power generation mechanism can include solar cells orpanels. The power source can include a battery or capacitive device. Themechanism to control the heating and cooling of the relay satellite 420may control the heating and cooling of the relay satellite 420passively, so the mechanism does not require a power source to functionproperly. The mechanism to point the transmitter or antenna to the Earthmay steer or rotate the position of the relay satellite 420 passively.

In one example, the ground receiver 440 can receive a first RF signaldirectly from the RF signal carrier 430. The RF signal carrier 430 cancommunicate a second RF signal to the relay satellite 420. The relaysatellite 420 can receive the second RF signal, shift a frequencyassociated with the second RF signal to create a third RF signal (i.e.,a transponded RF signal), and send the third RF signal to the groundreceiver 440. In addition, the third RF signal can be amplified withrespect to the second RF signal. As an example, the relay satellite 420can shift the frequency of the second RF signal in order to increase theDoppler shift associated with the second RF signal. The ground receiver400 can possess information on the position and ephemeris of the GPSsatellite 410, relay satellite 420 and RF signal carrier 430. In otherwords, the ground receiver 440 can know the orbit for each of the GPSsatellite 410, relay satellite 420 and RF signal carrier 430.

In addition, the relay satellite 420 can receive a GPS signal from theGPS satellite 410, shift a frequency of the GPS signal to create thethird RF signal (i.e., a transponded RF signal), and send the third RFsignal to the ground receiver 440. In addition, the third RF signal canbe amplified with respect to the second RF signal. As an example, therelay satellite 420 can shift the frequency of the GPS signal in orderto increase the Doppler shift associated with the GPS signal.

Therefore, the ground receiver 440 can receive a transponded RF signalfrom the RF signal carrier 430 (e.g., the third RF signal) via the relaysatellite 420 and/or receive the transponded RF signal from the GPSsatellite 410 (e.g., the third RF signal) via the relay satellite 420.In addition, the ground receiver 440 can have a line of sight to 12 ormore GPS satellites. The relay satellite 420 can transpond GPS signalsfrom one or more GPS satellites in addition to transponding the RFsignals from the RF signal carriers.

The ground receiver 440 can calculate a first range (R1) that representsa distance traveled by the first RF signal between the RF signal carrier430 and the ground receiver 440. The ground receiver 440 can calculate athird range (R3) that represents a distance traveled by the third RFsignal between the relay satellite 420 and the ground receiver 440. Asecond range (R2) can represent a distance between the RF signal carrier430 and the relay satellite 420. A GPS range (G1) can represent adistance traveled by the GPS signal between the GPS satellite 410 andthe relay satellite 420. The ground receiver 440 can use the first range(R1), the second range (R2), the third range (R3) and the GPS range (G1)to determine the geographical location of the ground receiver 440.

In one example, the ground receiver 440 can measure a time of flight (T)for the GPS signal to arrive at the ground receiver 440 from the GPSsatellite 410 by decoding or despreading the third RF signal todetermine a GPS ephemeris and GPS time. In other words, T can representthe amount of time for the GPS signal to be communicated from the GPSsatellite 410, transponded at the relay satellite 420, and then receivedat the ground station 440. The ground receiver 440 can correlate GPSchips to find the time of flight (T) from the GPS satellite 410 to theground receiver 440.

The ground receiver 440 can calculate an estimate of R3 using

${T = {\frac{{G\; 1} + {R\; 3}}{c} + d}},$wherein c is the speed of light and G1 is predetermined (e.g., using analmanac and it can be looked up by decoding the transponded GPS signal).In addition, d is a measured time delay representing an amount of timefor the relay satellite 420 to receive the GPS signal from the GPSsatellite 410 or receive the second RF signal from the RF signal carrier430, shift a frequency associated with the GPS signal or the second RFsignal to create the third RF signal, and send the third RF signal tothe ground receiver 440. In other words, the ground receiver 440 cancalculate the time of flight (T) and then subtract a known time ofprorogation from the GPS satellite 410 to the ground receiver 440 (i.e.,

$\frac{{G\; 1} + {R\; 3}}{c}$and transponder delay (d). An estimated range from the relay satellite420 to the ground receiver 440 (i.e., R3) can be slightly in error if areceiver clock differs from the GPS time. After two or more measurementsat different orbital positions of the relay satellite 420, the groundreceiver 440 can solve receiver clock bias and compute an absolute rangefrom the relay satellite 420 to the ground receiver 440.

The ground receiver 440 can calculate R1 using

${{\frac{R\; 1}{c} + \tau} = {\frac{{R\; 3} + {R\; 2}}{c} + d}},$wherein t represents a time lag between the ground receiver 440receiving the third RF signal from the relay satellite 420 with respectto receiving the first RF signal from the RF signal carrier 430, whereinR2 is predetermined (e.g., using an almanac). Therefore, the groundreceiver 440 can calculate R1 and R3. The ground receiver 440 canpreviously identify G1 and R2. The ground receiver 440 can use R1, R2,R3 and G1 when determining its geographical position. In one example, atleast four total measured Doppler or computing range (R3) signals areused to find X, Y and Z coordinates of the ground receiver 440, as wellas the oscillator offset.

In one configuration, the ground receiver 440 can calculate a firstDoppler shift associated with the first RF signal (i.e., a direct sourcesignal) received at the ground receiver 440 from the RF signal carrier430. The ground receiver 440 can calculate a second Doppler shiftassociated with the third RF signal received at the ground receiver 440from the relay satellite 420 (i.e., a transponded source signal),wherein the third RF signal has an increased Doppler shift with respectto the second RF signal and the GPS signal. The ground receiver 440 candetermine its geographical location using the first Doppler shift, thesecond Doppler shift, the first range (R1), the second range (R2), thethird range (R3) and the GPS range (G1).

FIG. 5 illustrates a diagram of a ground receiver 540 receiving radiofrequency (RF) signals directly from an RF signal carrier 530 andtransponded RF signals from the RF signal carrier 530 via a relaysatellite 520 in order to determine a geographical location of theground receiver 540. The RF signals can be non-GPS signals (e.g., RFsignals intended for weather, communication, or research). In otherwords, the non-GPS signals can be non-ranging RF signals. The RF signalcarrier 530 can include a satellite operated by a foreign entity or anon-GPS satellite. In one example, the RF signal carrier 530 can be aGEO satellite. Alternatively, the RF signal carrier 530 can be a LEOsatellite or MEO satellite. In addition, the relay satellite 520 can bea cube satellite (CUBESAT), pico-satellite, nano-satellite,micro-satellite, or other similar types of small and/or inexpensivesatellites.

The ground receiver 540 can receive a first RF signal from the RF signalcarrier 530. The ground receiver 540 can receive a third RF signal fromthe relay satellite 520, wherein the third RF signal is a transponded RFsignal. In other words, the relay satellite 520 can receive a second RFsignal from the RF signal carrier 530 and shift a frequency of thesecond RF signal to create the third RF signal. In one example, therelay satellite 520 may shift the frequency in order to increase aDoppler shift associated with the second RF signal when, for example,the RF signal carrier 530 is a GEO satellite.

In one example, a first range (R1) can represent a distance traveled bythe first RF signal between the RF signal carrier 530 and the groundreceiver 540. A second range (R2) can represent a distance between theRF signal carrier 530 and the relay satellite 520. A third range (R3)can represent a distance traveled by the third RF signal between therelay satellite 520 and the ground receiver 540.

The ground receiver 540 can determine its geographical location usingthe first range (R1), the second range (R2), and the third range (R3).In particular, the ground receiver 540 can determine its geographicallocation using

${{\frac{R\; 1}{c} + \tau} = {\frac{{R\; 3} + {R\; 2}}{c} + d}},$wherein c is a speed of light and R2 is predetermined (e.g., using analmanac). In addition, d is a measured time delay representing an amountof time for the relay satellite 520 to receive the second RF signal fromthe RF signal carrier 530, shift a frequency associated with the secondRF signal to create the third RF signal, and send the third RF signal tothe ground receiver 540. Furthermore, t represents a time lag betweenthe ground receiver 540 receiving the third RF signal from the relaysatellite 520 with respect to receiving the first RF signal from the RFsignal carrier 530. In one example, R1-R3 is a constant and R3 isconditional on R1.

FIG. 6 depicts a flow chart of a method 600 for determining ageographical location of a ground receiver. A plurality of radiofrequency (RF) signals can be received from a plurality of RF signalcarriers at the ground receiver, wherein the plurality of RF signalcarriers include at least one non-dedicated satellite, wherein the atleast one non-dedicated satellite is operated by a foreign entity or isa non-global positioning system (non-GPS) satellite, as in block 610. ADoppler shift associated with each of the plurality of RF signals at theground receiver can be measured, as in block 620. The geographicallocation of the ground receiver can be determined in X, Y and Zcoordinates based in part on the Doppler shift associated with each ofthe plurality of RF signals, as in block 630.

FIG. 7 depicts a flow chart 700 of an additional method for determininga geographical location of a ground receiver. A first radio frequency(RF) signal can be received, at the ground receiver from an RF signalcarrier, wherein the RF signal carrier includes at least onenon-dedicated satellite, wherein the at least one non-dedicatedsatellite is operated by a foreign entity or is a non-global positioningsystem (non-GPS) satellite, as in block 710. A third RF signal can bereceived, at the ground receiver from a relay satellite, wherein thethird RF signal is a transponded RF signal, wherein the relay satellitereceives a GPS signal from a GPS satellite and shifts a frequency of theGPS signal to create the third RF signal or the relay satellite receivesa second RF signal from the RF signal carrier and shifts a frequency ofthe second RF signal to create the third RF signal, as in block 720. Afirst range (R1) can be calculated that represents a distance traveledby the first RF signal between the RF signal carrier and the groundreceiver, as in block 730. A third range (R3) can be calculated thatrepresents a distance traveled by the third RF signal between the relaysatellite and the ground receiver, wherein a second range (R2)represents a distance between the RF signal carrier and the relaysatellite and a GPS range (G1) represents a distance traveled by the GPSsignal between the GPS satellite and the relay satellite, as in block740. The geographical location of the ground receiver can be determinedbased in part on the first range (R1), the second range (R2), the thirdrange (R3) and the GPS range (G1), as in block 750.

FIG. 8 depicts functionality of computer circuitry of a ground receiver800 operable to determine a geographical location of the groundreceiver. The computer circuitry can be configured to receive a firstradio frequency (RF) signal, at the ground receiver from an RF signalcarrier, as in block 810. The computer circuitry can be configured toreceive a third RF signal, at the ground receiver from a relaysatellite, wherein the third RF signal is a transponded RF signal,wherein the relay satellite receives a second RF signal from the RFsignal carrier and shifts a frequency of the second RF signal to createthe third RF signal, as in block 820. The computer circuitry can befurther configured to calculate a first range (R1) that represents adistance traveled by the first RF signal between the RF signal carrierand the ground receiver, as in block 830. In addition, the computercircuitry can be configured to calculate a third range (R3) thatrepresents a distance traveled by the third RF signal between the relaysatellite and the ground receiver, wherein a second range (R2)represents a distance between the RF signal carrier and the relaysatellite, as in block 840. Furthermore, the computer circuitry can beconfigured to determine the geographical location of the ground receiverbased in part on the first range (R1), the second range (R2), and thethird range (R3), as in block 850.

Various techniques, or certain aspects or portions thereof, may take theform of program code (i.e., instructions) embodied in tangible media,such as floppy diskettes, CD-ROMs, hard drives, non-transitory computerreadable storage medium, or any other machine-readable storage mediumwherein, when the program code is loaded into and executed by a machine,such as a computer, the machine becomes an apparatus for practicing thevarious techniques. In the case of program code execution onprogrammable computers, the computing device may include a processor, astorage medium readable by the processor (including volatile andnon-volatile memory and/or storage elements), at least one input device,and at least one output device. The volatile and non-volatile memoryand/or storage elements may be a RAM, EPROM, flash drive, optical drive,magnetic hard drive, or other medium for storing electronic data. Thesatellite may also include a transceiver module, a counter module, aprocessing module, and/or a clock module or timer module. One or moreprograms that may implement or utilize the various techniques describedherein may use an application programming interface (API), reusablecontrols, and the like. Such programs may be implemented in a high levelprocedural or object oriented programming language to communicate with acomputer system. However, the program(s) may be implemented in assemblyor machine language, if desired. In any case, the language may be acompiled or interpreted language, and combined with hardwareimplementations.

It should be understood that many of the functional units described inthis specification have been labeled as modules, in order to moreparticularly emphasize their implementation independence. For example, amodule may be implemented as a hardware circuit comprising custom VLSIcircuits or gate arrays, off-the-shelf semiconductors such as logicchips, transistors, or other discrete components. A module may also beimplemented in programmable hardware devices such as field programmablegate arrays, programmable array logic, programmable logic devices or thelike.

Modules may also be implemented in software for execution by varioustypes of processors. An identified module of executable code may, forinstance, comprise one or more physical or logical blocks of computerinstructions, which may, for instance, be organized as an object,procedure, or function. Nevertheless, the executables of an identifiedmodule need not be physically located together, but may comprisedisparate instructions stored in different locations which, when joinedlogically together, comprise the module and achieve the stated purposefor the module.

Indeed, a module of executable code may be a single instruction, or manyinstructions, and may even be distributed over several different codesegments, among different programs, and across several memory devices.Similarly, operational data may be identified and illustrated hereinwithin modules, and may be embodied in any suitable form and organizedwithin any suitable type of data structure. The operational data may becollected as a single data set, or may be distributed over differentlocations including over different storage devices, and may exist, atleast partially, merely as electronic signals on a system or network.The modules may be passive or active, including agents operable toperform desired functions.

Reference throughout this specification to “an example” or “exemplary”means that a particular feature, structure, or characteristic describedin connection with the example is included in at least one embodiment ofthe present invention. Thus, appearances of the phrases “in an example”or the word “exemplary” in various places throughout this specificationare not necessarily all referring to the same embodiment.

As used herein, a plurality of items, structural elements, compositionalelements, and/or materials may be presented in a common list forconvenience. However, these lists should be construed as though eachmember of the list is individually identified as a separate and uniquemember. Thus, no individual member of such list should be construed as ade facto equivalent of any other member of the same list solely based ontheir presentation in a common group without indications to thecontrary. In addition, various embodiments and example of the presentinvention may be referred to herein along with alternatives for thevarious components thereof. It is understood that such embodiments,examples, and alternatives are not to be construed as defactoequivalents of one another, but are to be considered as separate andautonomous representations of the present invention.

Furthermore, the described features, structures, or characteristics maybe combined in any suitable manner in one or more embodiments. In thefollowing description, numerous specific details are provided, such asexamples of layouts, distances, network examples, etc., to provide athorough understanding of embodiments of the invention. One skilled inthe relevant art will recognize, however, that the invention can bepracticed without one or more of the specific details, or with othermethods, components, layouts, etc. In other instances, well-knownstructures, materials, or operations are not shown or described indetail to avoid obscuring aspects of the invention.

While the forgoing examples are illustrative of the principles of thepresent invention in one or more particular applications, it will beapparent to those of ordinary skill in the art that numerousmodifications in form, usage and details of implementation can be madewithout the exercise of inventive faculty, and without departing fromthe principles and concepts of the invention. Accordingly, it is notintended that the invention be limited, except as by the claims setforth below.

What is claimed is:
 1. A ground receiver operable to determine ageographical location of the ground receiver, the ground receiver havingcomputer circuitry configured to: receive a plurality of radio frequency(RF) signals from a plurality of RF signal carriers at the groundreceiver, wherein the plurality of RF signal carriers include at leastone non-dedicated satellite, wherein the at least one non-dedicatedsatellite is a non-global positioning system (non-GPS) satellite;measure a Doppler shift associated with each of the plurality of RFsignals at the ground receiver, wherein the computer circuitry isfurther configured to: measure frequencies for each of a plurality ofnon-spread spectrum RF signals; compare the frequencies to knownfrequencies associated with the plurality of non-spread spectrum RFsignals; calculate a difference between the frequencies that aremeasured and the known frequencies in order to calculate the Dopplershift for each of the non-spread spectrum RF signals; and determine thegeographical location of the ground receiver in X, Y and Z coordinatesbased in part on the Doppler shift associated with each of the pluralityof RF signals.
 2. The ground receiver of claim 1, wherein the RF signalcarriers comprise at least one of low Earth orbit (LEO) satellites,medium Earth orbit (MEO) satellites or geostationary (GEO) satellites.3. The ground receiver of claim 1, wherein the RF signals comprise RFsignals other than global positioning system (GPS) signals or rangingsignals.
 4. The ground receiver of claim 1, wherein the computercircuitry of the ground receiver is further configure to determine anoscillator offset associated with the ground receiver, wherein theoscillator offset includes a frequency drift in a local oscillator ofthe ground receiver.
 5. The ground receiver of claim 1, wherein thecomputer circuitry of the ground receiver is further configure todetermine, at the ground receiver, source locations associated with theplurality of RF signal carriers and the frequencies associated with theplurality of RF signals prior to determining the geographical locationof the ground receiver.